1. Field of the Invention
The present invention relates to an optical disc substrate and a manufacturing method of an optical disc master for manufacturing the optical disc substrate.
2. Description of the Prior Art
There existed various types of optical discs such as a read only type, a recordable type and a rewritable type. An optical disc, which is provided with a read only area and a recordable area enabling to be recorded by a user in one substrate, has been proposed and practiced. In an optical disc having a pit area to be a read only area and a groove area to be a recordable area, generally, a pit depth is set to xcex/4n, which is the maximum amplitude of a sum signal of a reproduced signal, wherein xcex is a wavelength of a laser beam for recording and reproducing and xe2x80x9cnxe2x80x9d is a refractive index of a substrate of the optical disc. On the other hand, a groove depth is set to xcex/8n, which is the maximum amplitude of a differential signal of a tracking signal. In order to obtain such an optical disc substrate as being formed in one surface with a deep pit and a shallow groove, which are different in depth from each other, it is necessary for a pit and a groove to be varied in depth while forming a photoresist pattern as one process of manufacturing an optical disc master, which is a foundation of manufacturing the optical disc substrate.
Generally, in a process of forming such a photoresist pattern, there existed the method of changing each depth of pit and groove in one substrate such that using a laser beam in stronger intensity forms a pit and using a laser beam in weak intensity forms a groove. A depth of pit is determined by a film thickness of photoresist coated on a surface of substrate. However, a depth of groove depends on an intensity of laser beam and is apt to vary by a change of laser beam intensity or focus signal. A width of groove also depends on laser beam intensity, so that it is extremely hard to obtain an arbitrary width of groove. Further, a shape of groove obtained becomes approximately a V shaped one, so that a stable tracking signal can hardly be obtained. Furthermore, a recording power for recording a signal in a groove is necessary to be more powerful than that for recording in a conventional groove having a rectangular bottom, so that such a V shaped groove is a shape of unsuitable for recording in higher liner velocity.
Accordingly, in consideration of the above-mentioned problems of the prior art, an object of the present invention is to provide an optical disc substrate and a manufacturing method of the optical disc substrate and an optical disc master provided with a shape of land pre-pit being low in error, wherein a reproduction signal would not be affected by a land pre-pit or a groove adjacent to the land pre-pit when forming a pit and a groove in different depth in one master by using the plasma etching and ashing processes.
In order to achieve the above object, the present invention provides, according to a first aspect thereof, an optical disc substrate comprising: a pit area having pits deep in depth; a groove area having grooves shallow in depth; and a land pre-pit provided between the grooves in one substrate, wherein a depth of a section of one groove allocated inside the land pre-pit in the radial direction is shallower than each depth of grooves surrounding the section of the groove, and wherein a center of the land pre-pit is shifted to the inner circumference direction with respect to the radial direction.
According to a second aspect of the present invention, there provided a manufacturing method of an optical disc master for manufacturing a glass made optical disc master comprising a pit area having pits deep in depth, a groove area having grooves shallow in depth and a land pre-pit provided between the grooves, the manufacturing method comprising steps of: coating photoresist on a glass substrate; forming a pit section by irradiating on the pit area a laser beam having a first power exposing the photoresist as deep as a surface of the glass substrate; forming a groove section shallow in depth by irradiating on the groove area a laser beam having a second power lower than the first power so as not to reach to the surface of the glass substrate; and forming a land pre-pit section on a land existing between the groove section at a position displaced from a centerline of the land by a predetermined amount in the radial direction by irradiating the laser beam having the second power, wherein the laser beam having the second power is changed over to a third power lower than the second power while forming the groove section adjacent to the land pre-pit section, the manufacturing method further comprising steps of: etching only the pit section as deep as a predetermined depth through a plasma etching process; ashing the photoresist as far as a bottom of the land pre-pit reaches to the surface of the glass substrate through an ashing process; forming the pits, the grooves and the land pre-pit by etching the pit section, the groove section and the land pre-pit section so as to become respective predetermined depths through the plasma etching process; and removing the photoresist through the ashing process.
According to a third aspect of the present invention, there provided an optical disc substrate comprising: a first pit area having pits deep in depth; a second pit area having pits shallow in depth; a groove area having grooves shallow in depth and a land pre-pit provided between the pits shallow in depth in one substrate, wherein a depth of a section of one pit allocated inside the land pre-pit in the radial direction is shallower than each depth of pits surrounding the section of the pit, and wherein a center of the land pre-pit is shifted to the inner circumference direction with respect to the radial direction.
According to a fourth aspect of the present invention, there provided a manufacturing method of an optical disc master for manufacturing a glass made optical disc master comprising a first pit area having deep pits, a second pit area having shallow pits, a groove area having shallow grooves and a land pre-pit provided between the shallow pits, the manufacturing method comprising steps of: coating photoresist on a glass substrate; forming a deep pit section by irradiating on the first pit area a laser beam having a first power exposing the photoresist as deep as a surface of the glass substrate; forming a shallow pit section by irradiating on the second pit area a laser beam having a second power lower than the first power so as not to reach to the surface of the glass substrate; and forming a land pre-pit section on a land existing between the shallow pit section at a position displaced from a centerline of the land by a predetermined amount in the radial direction by irradiating the laser beam having the second power, wherein the laser beam having the second power is changed over to a third power lower than the second power while forming the shallow pit section adjacent to the land pre-pit section, the manufacturing method further comprising steps of: etching only the deep pit section as deep as a predetermined depth through a plasma etching process; ashing the photoresist as far as a bottom of the land pre-pit reaches to the surface of the glass substrate through an ashing process; forming the groove, the shallow pits, the deep pits and the land pre-pit by etching the groove section, the shallow pit section, the deep pit section and the land pre-pit section so as to become respective predetermined depths through the plasma etching process; and removing the photoresist through the ashing process.
Other object and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.